The impregnation of prefabricated carrier bodies affords various advantages over the production of a catalyst in a single process stage, in which the carrier material and the active materials are mixed and are subsequently shaped to catalyst bodies.
The prefabricated carrier bodies usually consist of SiO.sub.2 and can be produced without restriction as to the active substances. As a result, carrier bodies that are uniform in composition and have a large internal surface area and a high mechanical strength can be made. The impregnation results in a uniform distribution of the active substances throughout the carrier bodies so that the resulting catalyst bodies are uniform in composition and rejection of poor quality products is avoided. The catalysts have high conversion activities.
The production of catalysts by an impregnation of prefabricated carrier bodies has been described in commonly assigned U.S. Pat. No. 4,431,573 of Feb. 14, 1984 and corresponding German patent document - open application DE-OS No. 30 33 319. The carrier bodies in this system are impregnated with an impregnating solution which contains 600 to 1100 grams H.sub.2 SO.sub.4 and 140 to 830 grams V.sub.2 O.sub.5 and alkali metal sulfate per liter. The impregnation is effected at a temperature between 60.degree. C. and the boiling temperature of the impregnating solution. The impregnated carrier bodies are dried with agitation at a temperature of up to 700.degree. C. The catalysts have high conversion activities even at low temperatures and are mechanically stable even at high temperatures.
German Application No. 30 22 894 describes such a process for the production of catalysts for use in a fluidized bed. In that process the carrier bodies do not contain Al.sub.2 O.sub.3 in excess of 4% and are impregnated with a solution which has a low sulfuric acid content and contains vanadyl oxysulfate and potassium hydrogen sulfate. In this solution vanadium is present in tetravalent form. The impregnation is followed by drying at a temperature of 150.degree. to 250.degree. C. The carrier bodies or the impregnated carrier bodies are hardened at a temperature which is not in excess of 600.degree. C. and preferably between 200.degree. and 400.degree. C.
Catalysts may also be produced in a two-stage process comprising an impregnation with alkaline or neutral solutions (German Patent Publication No. 12 81 407, German patent document - open application DE-OS No. 15 42 177, Austrian Pat. No. 116,361, U.S. Pat. No. 1,941,426; B. Waeser "Die Schwefelsaurefabrikation", 1961, Verlag Friedrich Vieweg und Sohn, Brunswick, Germany, pages 252 to 258). This process, however, involves an attack of the alkaline solution on the SiO.sub.2 content of the carrier bodies. If neutral solutions are employed, the poor solubility of the active substances requires an impregnation in a plurality of stages so that it is difficult to adjust a uniform content of active substances.